Changes in body composition, in particular the onset of obesity, may result from reductions in total daily energy expenditure (TDEE) as a consequence of relative physical inactivity. Children previously treated for acute lymphoblastic leukemia (ALL) become obese, yet the mechanism remains undefined. TDEE and physical activity levels [PAL = TDEE/basal metabolic rate(BMR)] were measured in 34 long-term survivors of ALL and compared with results from 21 survivors of other malignancies and 32 healthy sibling control subjects using the flex-heart rate technique. Body composition was measured by dual energy x-ray absorptiometry. The median TDEE was reduced in the ALL group(150 kJ·kg d-1) compared with other malignancies and controls(207 and 185 kJ·kg d-1, respectively,p< 0.01). This reduction was accounted for mainly by a relative decrease in the PAL of the ALL group (1.24) compared with both other malignancies and controls (1.58 and 1.47, respectively,p< 0.01). TDEE and PAL were correlated with percentage body fat (r= -0.39,p< 0.001 andr= -0.24,p< 0.05, respectively). Obesity in survivors of ALL may, in part, be explained by a reduction in TDEE as a consequence of reduced PAL. The cause of such reduction is uncertain.Abbreviations: ALL,acute lymphoblastic leukemia;BMI,body mass index;BMR,basal metabolic rateCI,confidence interval;DEXA,dual energy x-ray absorptiometry;FFM,fat free mass;HR,heart rate;PAL,physical activity level;RMR,resting metabolic rate;TDEE,total daily energy expenditure;NHL,non-Hodgkin's lymphoma;SDS,SD score